This invention relates to a method of preparing a sheet of a lignocellulosic material for the manufacture of a finished product and to a method of forming an article or finished product from one or more sheets of a lignocellulosic material so treated.
It is well known to manufacture spirally or convolutely wound paper tubing or angles, the latter referred to as edge board, from kraft paper. Such products are most frequently made by first cutting large reels of kraft paper into reels of limited widths of from 40 mm to 160 mm, and then unwinding the paper from these reels or "biscuits" for multi layer lamination, either over a mandrel in the case of spirally wound paper tubing, or across formers in the case of edge board, the glue between the layers being generally a water based polyvinyl acetate or alkali silicate. The product comes off the line continuously and can then be cut to lengths of any size. The result is a product that is fit for the purposes for which it is presently manufactured.
However, these basic shapes, i.e. tubular or angular sections, are generally not suitable for use in other applications such as high pressure pipes, ventilation pipes, replacements for aluminium extrusions, as poles or posts to hold signs, as fencing poles or posts, as irrigation pipes, or for a host of other applications where the tubular or angular sections are subjected to a greater degree of mechanical or other stress. In these cases, the paper from which the tubular or angular sections are made, must first be modified in order to provide the required degree of water resistance and mechanical strength.
In the manufacture of sheet products, i.e boards or panels, from a lignocellulosic material, it is also necessary to provide the material with the required degree of water resistance and mechanical strength.
Natural fibrous materials or lignocellulosic materials are comprised of hemi celluloses, celluloses and lignin. During the paper making process, the lignins are digested out of the composition, leaving principally the celluloses and hemi celluloses. A change in the moisture content of these materials results in swelling, as a result of their hydrophilic nature, and thus a loss in strength. The reason is that the cell wall polymers of the materials contain hydroxyl or other oxygen containing groups that attract water through hydrogen bonding. It is the hemi celluloses which are the most hygroscopic. Water can give rise to further degradation as a result of attack by micro organisms.
It is known to modify lignocellulosic materials chemically. Various classes of chemical reactions have been used with wood products and these are esters, acetals and ethers produced inter alia by the use of anhydrides.
Examples of documents which teach the use of anhydrides include U.S. Pat. Nos. 4,832,987, 5,055,247 and 5,064,592.
Examples of documents which teach the use of anhydrides to treat a cellulosic material to which there is then applied a polymer, are U.S. Pat. Nos. 5,120,776; 5,385,754 which teaches a process for modifying lignocellulosic material by a chemical treatment method, which method comprises treating the lignocellulosic material with phthalic anhydride and a thermosetting resin selected from phenol formaldehyde resins, urea formaldehyde resins and urethane resins and then curing the phthalalated resin treated product so formed; CA 119:227002; and CA 120:135905.
However, it is undesirable when preparing an article using a glue bond to use a material that gives rise to toxic volatiles or that may be toxic in itself. It is undesirable to have flammable solvents present, and in gluing paper laminates together that have been chemically modified or resinated, solvent or water carriers cannot be accommodated in the adhesive composition, because it is not possible for them subsequently to escape from the composite.
European Patent 0390536 to Plascon Technologies (Pty) Limited teaches the impregnation of spirally wound paper tubing with a thermosetting resin in an extending liquid. However, it should be noted that the impregnation of a pre-formed pipe has a number of disadvantages. In the first case, very little mass of pipe can be fitted into a vacuum/pressure/vacuum impregnation cylinder and therefore efficiencies are low; the adhesive used in the pre-winding of the pipe may be inferior for the end purpose, such as the water based polyvinyl acetates and alkali silicates; and the pre-applied adhesive mitigates against the through penetration of the impregnating composition. The advantage of pre-impregnating narrow reels or biscuits of paper or other lignocellulosic material is that the volume of the treatment cylinder can be utilised to its maximum theoretical extent, and the subsequent winding of the pre-impregnated material in which the resin may be in the "B Stage"--in other words, not necessarily fully polymerised, allows the specification of the hot melt adhesive to be devoid of solvents or water that could be trapped in the tube composite, and the resins chosen may cross link with the resin in the paper, forming an intimate bond and a cohesive whole to the composite.
U.S. Pat. No. 4,505,778 teaches a paper composition comprising cellulosic fibres chemically bonded with polyisocyanate sizing resins consisting essentially of a blend of an aromatic polyisocyanate resin containing from 1 to 10% by weight of an isocyanate terminated prepolymer having the formula RO(CH.sub.2 CHR'O).sub.n CONHX wherein R is selected from an alkyl group containing one to four carbon atoms and a polyester condensation of a diacid and a polyether glycol, R' is selected from H or CH.sub.3, n is an integer ranging from 5 to 120, and X is a residue of an aromatic di or polyisocyanate, and which contains at least one free isocyanate group. The polyisocyanate composition is applied as an aqueous emulsion to the cellulosic fibres which are then formed into paper. Alternatively, an aqueous emulsion of the polyisocyanate may be applied to the surface of the paper after its formation. It is to be emphasized that the polyisocyanate is purely used as a sizing resin and that it is always applied in an aqueous solvent or emulsion.
U.S. Pat. No. 5,280,097 discloses a laminated product which has an organic polymer layer on at least one surface of a substrate of cellulosic material impregnated with polyisocyanate. The isocyanate resin may be dissolved in a suitable miscible organic solvent, preferably one such as propylene carbonate having a high boiling point and a low flammability and toxicity.
U.S. Pat. No. 5,008,359 discloses a method of making a cellulose based polymeric material and the products of the invention. A cellulosic substrate is impregnated with an essentially uncatalysed polyfunctional isocyanate and pressed at elevated temperature and pressure. The polyisocyanate may be impregnated into the cellulosic material in either neat form or in solution with a solvent such as acetone.
U.S. Pat. No. 3,666,593 teaches a process for making a paper-overlaid panel which comprises providing a substrate and at least one layer of kraft paper, applying an organic polyisocyanate resin to the paper, and pressing the layer of paper against the substrate under closed conditions, whereby the formed carbon dioxide will be entrapped causing internal pressure to impregnate the resin into the paper and into the substrate and to bond the paper to the substrate. The polyisocyanate resin may be diluted with a solvent.
There is therefore a need for products made from lignocellulosic sheets in which the sheets have first been chemically modified and resinated and which are then laminated subsequently.